Color image screen utilizing electroluminescence



M. v. KALFAIAN 2,872,613

COLOR IMAGE SCREEN UTILIZING ELECTROLUMINESCENCE 2 Sheets-Sheet 2 FiledSept. 7. 1955 38 TRAN51. UCENT ELECTROLUHINESCENT I- TRIPS TRl-COLORPHOSPHOR 39 STRIPES Acz PLATE BARRIER .sc EEN secozvn lL, smsslvsSURFACE s /r I j come-c1 P {g W RED a EN INVENTOR- COLOR IMAGE SCREENUTILIZING ELECTROLUMINESCEN CE Meguer V. Kalfaian, Los Angeles, Calif.

Application September 7, 1955, Serial No. 532,852 2 Claims. (c1. 315-12This invention relates to television systems and is particularlyconcerned with devices for electro-optical image formation in naturalcolors utilizing electroluminescence as the source of opticalillumination. Its main object is to provide an electroluminescent colorscreen, which may be activated by an electron beam for the production ofimages in natural colors. Another object is to provide anelectroluminescent color screen, the primaries of which may beselectively activated within the spot-area of a single electron beam, ineither simultaneous or sequential additive systems, without altering thedirection of the beam from its normal deflection of forming a scanningraster on the image screen. t Production of light by electroluminescencehas been known previously, and its practicality is shown by Mager in U.S. Patent No. 2,566,349, September 4, 1951. The electroluminescentcells, or luminous capacitors as they may be called, consist of parallelplate electric conductors with a suitable electroluminescent phosphordispersed therebetween. Normally, one of the conductors is madetranslucent, whereby the light emitted by the phosphor cells may passtherethrough. The light emission of these phosphor'cells is effected byan alternating electric field passing through them, as for example, bythe application of alternating voltage upon the two conductor platesfrom an outside source. The intensity of light given oft by anelectroluminescent cell is a function of the applied voltage as well asthe frequency of the applied voltage, but as the frequency is increaseda point is reached where higher frequencies do not increase theintensity of light emission. This limitation may be attributed to theslow transfer of energy from the electric field to the activator centersof presently available electroluminescent phosphor cells; but moresuitable phosphor materials may be found for high frequency excitationwith high efiiciency.

The thickness of the electroluminescent screen is usually made betweento 100 microns thick, and therefore, it

is possible to concentrate the luminescence into a very small spot areabetween two conductors of this size without causing objectionable coronaof the electric field between the two conductors. It is accordinglypossible to place the electroluminescent screen between first and secandplurality of spaced parallel strip-like conductor electrodes, runningalong lines of 90 degree relation with respect to each other, andluminesce any spot area of the screen where two of the first and secondstrip-like electrodes intersect each other, by applying the requiredpotential to them. When these first and second spaced parallelstrip-like electrodes are excited sequentially, the entire surface ofthe electroluminescent screen may be luminesced elementally, andfurther, when these sequential excitations are varied in magnitudesaccording to image forming signals, then a completeelectro-optical imagemay be formed on the composite screen. This type of image formingelectroluminescent screen had been disclosed byPiper in U. S. Patent No.2,698,915, January 4, 1955. The objection to this type of image forming,W the difiiculty of seaming, where,

F atented Feb. 3, 1959 either the first and second spaced parallelstrip-like electrodes are excited sequentially by sluggish mechanicalmeans, or an electronic computer arrangement, which becomesprohibitively complicated for practical uses. "In

.order to obviate these difiiculties, I had shown in my copendingapplication Serial No. 434,157 filed June 3, 1954, now Patent No.2,728,815, December 27, 1955, and Serial No. 481,174 filed January 11,1955, now Patent No. 2,813,223, November 12, 1957, an arrangement ofelectroluminescent target which may be activated in elemental areas by ascanning electron beam. The present invention includes improvements ofsuch an arrangement, a detailed specification of which is given in thefollowing in connection with the accompanying drawings, wherein: Fig. lis a longitudinal section of the color image tube, showing onlyessential parts, which includes the electroluminescent image target incross-sectional view;

Fig. 2 is an exploded view of the color image target,

including schematically the essential parts by which the video signalsare applied to the target, and the direction in which the scanning beamforms a raster upon the image target;

Figs. 3 and 4 show diagrammatically different forms of adding barrierscreens to the image target in accordance with the invention;

Fig, 5 illustrates diagrammatically modification of the combined barrierand collector screens which is suitable for use in operating one form ofthis invention;

Fig. 6 is a cross-sectional view of the barrier screen, showing itsposition with respect to the image target; and

Fig. 7 is alongitudinal diagrammatic view of the color image tubeincorporating an electroluminescent color image forming target.

Fig. 1 illustrates the general layout of electroluminescent imageformatioruwherein, the screen comprises a translucent conductorelectrode 2; a layer of electroluminescent phesphor material I placedover the flat surface of electrode 2; and a mosaic of mutually insulatedconductor elements 3, 4, etc., placed over the phosphor material 1.Electrically, this screen structure forms a mosaic of capacitiveelements, for example, elements 2 and 3 forming one independentcapacitor, and elements 2 and 4 forming another independent capacitor,etc. Since as stated in the foregoing, that an electroluminescent screenmay be referred to as a luminous capacitorfit is obvious therefore, thatthe screen just described may be referred to as a screen comprising amosaic of luminous capacitors. The surfaces of elemental electrodes 3,4, facing the primary electron beam, are made to produce secondaryelectrons upon impact by the primary beam, the latter of which is madeto form a scanning raster upon the image target by conventional means,for example, .by the deflection yoke, as shown diagrammatically in thedrawing. A collector 5 of secondarily emitted electrons is included inthe arrangement of Fig. l. Functionally, when the beam-acceleratingfield is so adjusted that the ratio of secondary current to the primarycurrent is greater than unity, the potential .of bombarded electrode 3may be raised positive substantially equal to the potential of collectoranode 5, where an equilibrium state is reached, and the net secondaryemission is then substantially unity. At this point, when the potentialof electrode 2 is varied by the Video signal through resistance R, thepotential of electrode 3 will also vary through capacitive coupling, andeffect corresponding change in secondary emission, thus varying thecollector current. The effect therefore, is the same as if the electrode2 were acting as the control grid; the electrode 3 as the cathode; andthe collector 5 as the anode of an ordinary triode; causing the luminouscapacitor to charge and discharge according to the applied videovoltages. With the assumption that the thickness of dielectric sheet].is-less than the width of electrode 3, the transverse distribution ofelectric field between the electrodes 2 and 3 will be practicallyminimized, and the electroluminescence will and a raster formed by thescanning beam, they will "act as individual triodes, and produce thedesired lumines: cence according to the applied image signals. Whenchromatic images are to be reproduced, the electrode 2 may be dividedinto three mutually insulated interleaved sections, each section alignedwith a primary-color filter, and three separate video voltagesrepresenting different primary colors applied to'these sections.

As stated in the foregoing, electroluminescence is effected 'by changingelectric field across the luminous capa'citor. To achieve thiscondition, I had shown in my above mentioned first patent application,now Patent No. 2,728,815, Dec. 27, 1955, an arrangement of highfrequency carrier wave for the video signals, so that the electric fieldalternates several times across a luminous capacitorduring impingementof the scanning beam upon it. In another arrangement, as shown in myabove noted copending application (now patented as indicated by theabove given number), the required charge and discharge of a'lurninouscapacitormay be achieved'by changing the polarity of the 'video signalby a phase inverting flip fiop trigger circuit, which may be operated bythe frame blanking signals in a television system. As an alternative,the dielectric sheet may be made slightly conductive .to a value as todischarge the luminous capacitor during a frame or field period. Any ofthese systems for charging and discharging the luminous capacitors maybe incorporated with thepresent invention, and accordingly fur:therreference or drawing arrangement will not be repeated herein. Themain object of the present invention, however, is a modification of thetube structure, which will be described in detail in the followingspecification.

, Referring now'to the exploded view of a color image formingelectroluminescent screen in'Fig. 2, a color filtering film 7 is firstmounted over the inner surface of the face plate 6, which may serve asthe end wall of theimage tube, through which the image is viewed. Thecolor filtering film contains a large number of very narrow colorselective stripes, corresponding to suitable primary colors e. g., red(R), green (G), and blue (B). Next to each stripe, and alignedtherewith, is translucent conductor strip8. The conductor strips onstripes of each color are electrically connected to a correspondingresistor e. g., those on red stripes to resistor R1, those on bluestripes to resistorRZ, and those on green stripes to resistor R3. Nextover the conductor strips 8 is a sheet of thermoplastic dielectricmatrix 9, dispersed with phosphorescent luminous cells. Finally, amosaic of mutually insulated nescent phosphor film may be divided intonarrow stripes, aligned with the strips 8, and these stripes dispersedwith electroluminescent phosphor cells having characteristics ofluminescing in different primary colors, thus achieving similar resultsas if the color filtering film 7 were used. The elimination of the colorfiltering film 7 might be more desirable for practical purposes, as itabsorbs an objectionable portion of the light emission from theluminescent-film 9. These conditions, and also examples of phosphormaterials that might be utilized with the screen, as shown, have beendisclosed in my above mentioned copending applications (now patented asindicated above), and therefore, further specification is not necessaryherein.

As suggested in my above mentioned first patent application, now PatentNo. 2,728,815, Dec. 27,1955, the mosaic elements 10, in Fig. 2, do notnecessarily have to be metallic, or conductive. It has been showningeneral practice that someinsulating materials, for example, glass orplastics, have high secondarily emissive properties. Accordingly,the'uniform surface of the plastic material utilized as retainer of thephosphor cells .may as well replace the mosaicelements 10. In this case,the

' uniform surface of the plastic material, looking toward phosphorcells; a-secondary insulating material, having conductor elements 10eover'the dielectric sheet in the a region to be scanned by the electronbeam 11. In this arrangement, the resistors R1 to R3 are separatelyenergized by primary-color video signals arriving from separate sources,as designated by the letters R, G and B. Thus, when the high velocityelectron beam 11 strikes one or several of the elemental electrodes 10,the secondary emission establishes an electron path between theseelemental electrodes and the electrode strips 8 through electroncollector 12 and resistors R1 to R3, in the manner as described by wayof the arrangement given in Fig. 1. When this arrangement is to beutilized for monochrome image reproduction, the three terminals for R1,R2 and R3 are electrically connected in parallel, terminating to asingle resistor element, and monochrome image signals applied to it.Physically, however, the translucent strips 8 may be originally madeinto a single uniform sheet, and the color filtering film eliminated.Further, in reference to color image reproduction, the color filteringfilm maybe eliminated, and the uniform electrolumigreater secondarilyemissive properties, may be applied over-theoriginal dielectric sheet,for example, when the original dielectricis arranged with slightconductivity; or as an alternative, the electroluminescent material maybe first'applied on the base ofstrips 8, and secondarily emissiveinsulatingmaterial applied over, or, for example, thin sheet of mica orequivalents thereof, mounted over the electroluminescent material.

With the elimination of the mosaic elements 10, it is ,obvious that thescreen construction becomes physically simple, and further modificationsmaybe made without difficulty. For example, as suggested in myabovementioned copending application, Serial No. 481,174 filed Jan. 11, 1955,now Patent No. 2,813,223, Nov. 12, 1957, a barrier screen 19 may beutilized in conjunction with the image forming screen, as shown in Fig.3, for the purpose of avoiding re-distribution of the secondarilyemitted electrons over the emissive surface of the image forming target.In the'case of eliminating the mosaic electrodes 10, the barrier screengrid may be directly placed over the uniform-insulating surface of theimage screen,

such asshown in cross-sectional view in Fig. 6, wherein,

1-3, represents'the secondarily emissive insulating surface .of theelectroluminescent phosphor screen 31, and 14 represents the barriergrid screen. It is obvious from this assembly that less care is neededin aligning the barrier screen over the insulating surface, thanwhen'the mosaic electrodes '10 were used; although particular alignment.with respect to the strips 8 may be made, ifso desired, ac-

cording to the texture of the barrier grid, for example, it

may be made of parallel wires; a screen with square or round openings;or of any other shape; and lastly, as an alternative, the screen may beapplied upon the insulating emissive surface of 13 by known methods ofelectrodeposition, or equivalents thereof.

The functionof the barrier grid has been described elsewhere in previousliterature, but'as a brief reminder, thedrawingof Fig. 3-may beconsidered illustrative. =-Assuming that the velocity of the primaryelectron beam .15 is adjusted (by potential sources 16 and'17)toube-rhigh enough to cause secondary emission from the insulatingsurface of target, ls'greater than unity, the netefiectis 'to For chargeit positively. Due to the negative potential upon barrier screen 19 (bypotential 17) with'respect tothe accelerating wall 20, a retardingelectric field is set up between the .target 18 and the barrier screen19, such that, those secondary electrons having low velocity are unableto pass through the screen opening, and accordingly return to thepositive area of the target surface. Distribution of these low velocitysecondary electrons to adjacent surface areas of the target is avoidedby the fact that, the screen 19 is in close proximity with the emissivesurface of target 18 (such as shown in cross-sectional View in Fig. 6),and the slow secondaries are constrained to return to the same surfacearea within a screen mesh spacing before escaping it. Secondaryelectrons having high velocity, however, pass through the screenopenings and are further accelerated to the wall 20. As this secondaryescapement from the screen 19 continues, the potential upon thatparticular surface area of the target 18 increases until it issubstantially equal to the positive potential of accelerating wall 20,where an equilibrium state is reached and the net secondary current issubstantially equal to the primary current. The overall condition is thesame as described in the foregoing with reference to the illustrationsof Figs. 1 and 2, but in this case, the inclusion of the barrier screen19 avoids transverse distribution of elemental image luminescence uponthe image forming screen, which otherwise could be effected -by slowsecondary electrons emitted from the emitting surface of the target 18,or in Fig. 2.

In storage type of cathode ray tube, the inner wall is generallyutilized as the collecting anode of secondarily emitted electrons. Forthe purpose utilized herein, however, the image forming target area isconsidered to be very large, such as for large screen television imagereproduction. With such large target area, the distance from thegeometric center of the target to the inner wall of the tube isconsiderably large, and 'it is possible that some escaping electronsfrom the target may fall back to a remote area upon the target. To avoidthis condition, an auxiliary collector screen may be included in thetube, electrically connected to the main collector anode. This isschematically shown in Fig. 4, wherein, 21 is the primary electron beam;22 is the secondarily emissive target; 23 is the barrier screen; and 24is the auxiliary collector screen electrically connected to the maincollector anode 25. In this case, the primary beam passes through theauxiliary collector screen 24 (some of the primary electrons beingintercepted by this screen, but a major portion of it passingtherethrough), and reaches the target 22 in the previously describedmanner. However, those secondary electrons released from the centerportion of the target area find a shorter path to the screen 24, hencebeing attracted toward it, and those secondary electrons released fromthe edge of target area are (depending upon the distance between screen24 and barrier grid 23) partly attracted by the screen 24 and partly bythe wall 25, as shown by the dotted lines 26; thus further reducing thepossibility of re-distribution eifects.

An exploded diagrammatic view showing the respective relations of theseadded elements is given in Fig. 5, wherein, 27 is the secondarilyemissive target; 28 is the barrier screen; and 29 is the auxiliarycollector screen; the arrow 30 points the direction in which the primaryelectron beam travels. With regard to the foregoing description, theauxiliary collector screen 29 is desirable, but not necessarilyessential, and its physical form is not critical, since its purpose isto aid the main electron collector in collecting all the necessarysecondary electrons. For this reason, the screen 29 may have coarsermesh than the barrier screen 28, and it may also comprise few stretchedwires; thus avoiding appreciable amount of primary beam interception bythe screen 29.

Fig. 7 illustrates a cross-sectional diagrammatic view of the generalappearance of the electroluminescent color image forming tube, wherein:32 is the evacuated glass (or partly metal as the case may be) envelopeenclosing an electron-emitting cathode 33, adjacent to which is includedan intensity control electrode 34, followed; by a beam-forming structure35, which functions to concentrate the emitted electron stream 36 into asmall scanning spot projected upon the secondarily emissive surface 37of electroluminescent phosphor layer 38 which constitutes the imageforming target, including the rows of metal strips 39 directly mountedupon the inner surface of the viewing face plate 40 by a method, forexample, photodeposition, for convenience. Between the secondarilyemissive surface (mounted at right angles to the axis of the electronbeam 36) andthe beam 36 there is disposed the barrier grid 41 closelyadjacent to the surface 37. To complete the electron path of thesecondarily emitted electrons from target surface, there is included acollector anode 42 which may act both as collector of these secondarilyemitted electrons and accelerator of the primary beam 36, and to returnthe collected electrons to the emitter cathode 33 for the desiredcirculation of the primary beam. As shown cross-sectionally andschematically, every third of the translucent conductor strips 39 areelectrically connected in groups, to which the video signals indifferent primary colors (red, green, blue) are applied for the desiredcombination of color image formation upon the electroluminescent screen(in different primary color stripes) 38. As indicated in the foregoing,the phosphor screen may be of a single color, for example, white color,and a striped color film included between the inner surface of the faceplate 40 and the conductor strips 39. These variations, of course,depend upon the mode in which the image tube disclosed herein isutilized. Also, it had been indicated that the strips 39 could be allconnected in parallel electrically for the production of monochromeimages. It is thus obvious that the image tube described in theforegoing may be utilized for monochrome image formation; chromaticimage formation; or according to the particular use, the chromatic imageformation may be in two colors; three colors; or four colors withoutchanging the characteristic operation of the tube, or the systemassociated therewith. The image tube illustrated in Fig. 7 is includedwith a beam deflection coil 43 which is of the conventional typeutilized in monochrome television systems.

As has been experienced in the prior art, modifications andsubstitutions of parts in electronic devices of the type disclosedherein are possible without departing from the true spirit and scope ofthe invention, and accordingly, the foregoing should be construed onlyillustrative, and not in a limiting sense.

What I claim is:

1. An electroluminescent image forming device which comprises means forprojecting a primary electron beam; a target of image forming elementalluminous capacitors in the path of said beam comprising alight-transparent electrode disposed plane-perpendicular with respect tothe normal flow of said projected beam, an electroluminescent phosphorscreen overlying upon said electrode, facing the beam, and an array ofbeam responsive elements over the surface area of said screen, facingthe beam, each element adapted to produce secondary emission when struckby the primary beam, whereby forming elemental luminous capacitors, eachcapacitor comprising said light-transparent electrode, said screen, andone of said emissive elements; a collector anode for collecting theelectrons of secondary emission from any one of said elements underimpact by said beam, whereby forming elemental electrical paths betweensaid beam responsive elements and the collector; and an output terminalmeans coupled to said light-transparent electrode for applyingelectrical video signals of various intensities between said terminaland the collector for individual excitation of said luminous capacitorsin various intensities at points of primary impingement.

2. The device as set forth in claim 1, wherein, said light-transparentelectrode is divided into a plurality of adjacently positionedlight-transparent strip-like electrodes, and electrically connected insequential steps into first, second and third sections; and wherein saidphosphor screen comprises electroluminescent phosphors having thecharacteristics of exhibiting luminescence in first, second and thirdprimary colors in stripe-like sections adjacently aligned with saidstrip-like sections; and first, second and third output terminal meanscoupled to said first, second and third sections, respectively, forapplying first, second and third electrical video signals of variousintensities between last named terminals and said collector forindividual excitation of said luminous capacitors at points 5 saidelectron ernissive elements.

References Cited in the file of this patent UNITED STATES PATENTS Jensenet al 'Apr. 11, 1950 2,698,915 Piper Jan. 4, 1955 2,706,264 -AndersonApr. 12, 1955 2,728,815 Kalfaian Dec. 27, 1955

